The invention relates to a process for complete oxidation to sulphur of the H.sub.2 S present in a gas in a concentration of between 0.7% and 3% by volume. It also relates to the application of this process to the virtually total recovery, in the form of sulphur, of the sulphur compounds present in a residual gas from a sulphur plant.
The residual gases originating from a sulphur plant in which sulphur is produced by the Claus process for controlled oxidation of a sour gas containing H.sub.2 S, by means of a gas containing free oxygen, usually contain less than 4% by volume of sulphur compounds, a large proportion of which consists of H.sub.2 S, the remainder consisting of SO.sub.2, COS, CS.sub.2 and of sulphur vapour and/or vesicular sulphur.
Such residual gases are commonly treated in order to lower as much as possible the overall content of sulphur compounds with the aim of allowing them to be discharged to the atmosphere, after they have been burned, while conforming to the standards imposed by legislation relating to atmospheric pollution and simultaneously to recover these sulphur compounds in a form which contributes to increasing the yield of upgradable products formed from the sour gas treated in the sulphur plant.
Various processes for performing the treatment of a residual gas from a Claus sulphur plant are known, and especially processes comprising a combined hydrogenation and hydrolysis treatment of the residual gas with a view to converting the sulphur compounds which it contains into the form of H.sub.2 S alone, and then a cooling of the effluent resulting from the said combined treatment to an appropriate temperature and, lastly, a treatment of the cooled gaseous effluent in order to remove H.sub.2 S from it. This removal of H.sub.2 S can be carried out either by absorption of the H.sub.2 S by means of a selective solvent capable of being regenerated or else by controlled catalytic oxidation of the H.sub.2 S to sulphur.
One of the processes of the abovementioned type, which comprise a removal of H.sub.2 S by catalytic oxidation to sulphur, is the process described in citation FR-A-2702674, in which, after the combined hydrogenation and hydrolysis treatment of the residual gas originating from the sulphur plant, the gaseous effluent resulting from the treatment and containing H.sub.2 S as the only sulphur compound is cooled to a temperature between the dew point of the water in the said effluent and 180.degree. C. and the cooled gaseous effluent is brought, in the presence of a gas containing free oxygen, into contact with an oxidation catalyst promoting the selective oxidation of H.sub.2 S to sulphur, while operating at temperatures which, throughout the duration of the contact, are maintained at values taken between the said dew point and 180.degree. C. in order to oxidize the H.sub.2 S present in the gaseous effluent substantially quantitatively to sulphur and to deposit virtually all of the sulphur formed on the oxidation catalyst and to obtain a gas stream practically free from H.sub.2 S.
According to the invention a process is proposed for complete catalytic oxidation to sulphur of the H.sub.2 S present in a gas in a concentration of between 0.7% and 3% by volume, which constitutes an improvement to the catalytic oxidation stage of the process described in citation FR-A-2702674, in the sense that it makes it possible to control the temperature of the oxidation catalyst more efficiently and thus to maintain an optimum selectivity for sulphur during the catalytic oxidation of H.sub.2 S in contact with the oxidation catalyst. The substitution of the improved process for catalytic oxidation of H.sub.2 S according to the invention in the stage of catalytic oxidation of H.sub.2 S of the process for the treatment of residual gas of a sulphur plant, described in citation FR-A-2702674, results in an improved process for the treatment of a residual gas from a sulphur plant, the result being the possibility, for the combined unit for the treatment of a sour gas, associating the sulphur plant and the plant for the treatment of residual gas from this sulphur plant, using the abovementioned improved process, to reach overall sulphur recovery yields of at least 99.9%.